This invention relates to axial flow turbines, and in particular to axial flow turbines wherein the turbine also functions as a compressor.
Axial flow turbines are used to generate power for a variety of uses. These turbines commonly are composed of a compressor section which increases the pressure of the ambient air which enters the turbine, an ignition or flame area where fuel is injected into the compressed air, ignited and burned to add energy, and a turbine section where the expanding gas acts to generate mechanical power. The mechanical power is used to drive the compressor section and supply mechanical power for other purposes.
The common cycle used by the axial flow turbine is to compress ambient air, to heat the air by some means, and to use the expansion of this compressed air as a means for extracting mechanical energy. Commonly, the flow of the compressed air and heated gasses are in an essentially straight line path along the longitudinal length of the engine. It is well known that the efficiency of such engines is increased by increasing the pressure to which the air has been compressed and by heating the air to the greatest allowable temperature and passing this high temperature air through the turbines. The greatest temperature which can be used in such engines is determined by the material properties of the turbine portion of the engine. Since common engineering materials lose their mechanical properties when certain temperatures are exceeded, it is now necessary to limit the turbine temperatures to those determined by the properties of the turbine materials rather than to the temperatures desired for maximum efficiency. In modern turbine engines the temperature is generally limited by the introduction of excess air into the combustion system so as to reduce the temperatures of the gasses resulting from the combustion process. This results in less than maximum thermo-dynamic efficiency and in increased physical size of the turbine and high cost due to the use of exotic materials in the turbine section.
Other designers have proposed the use of a rotor that is common to both the compressor and turbine stages of the engine. Those designs allow cooling of the turbine blades during the compression stage so that the average temperature of the blades is reduced and thus allow for higher combustion temperatures. In those designs a single compression stage is used so that during approximately one-half of its revolution the rotor is exposed to the cooling air and during the remaining part of the revolution the rotor is exposed to the hot gasses.